Fluid operated actuator for movable members

ABSTRACT

A fluid operated actuator for controlling the movement of a member such as a valve and being connectable into a fluid circuit including pressure operated means for indicating extreme operative positions of the aforementioned member. The actuator including a piston-cylinder assembly, the piston of which is connectable to the aforementioned member, and pressure relief means operative to prevent the pressure acting on the piston reaching a predetermined level which is below that necessary to operate the indicating means. Cut-out means is provided within the piston-cylinder assembly to render the relief means inoperative when the piston reaches or approaches positions corresponding to the aforementioned extreme operative positions of the associated member, so that the pressure acting on the piston is able to rise sufficiently to operate the indicating means.

United States Patent n91 Cusveller June 3, 1975 i 1 FLUID OPERATEDACTUATOR FOR MOVABLE MEMBERS [75] Inventor: John Cusveller, MooncePonds.

Victoria, Australia [73] Assignee: Sperry Rand Australia Limited,

Victoria. Australia 22 Filed: Aug. 15.1972

211 Appl. No.: 280,879

Related US. Application Data [62] Division of Ser. No. 73,508, Sept. l8,1970. Pat. No.

[52] US. Cl. 251/3]; 91/1; 9l/408; 9l/409;9l/4l9; 9l/42019lf437',91/449; 92/1311 [51] Int. Cl. Flbk 31/12 [58] Field of Search..92/l3.6,l(14,5;9l/l, 91/392, 437, 408, 409, 420, 449, M9, 400, 402,404

[56] References Cited UNlTED STATES PATENTS l,009,664 ll/I9ll Hefty cta]. 91/402 l,749,924 3/l930 Runge ct al A 9l/402 2,292,336 8/l942Farnham 9l/408 2.410.978 ll/l946 9l/4l9 3,122,064 2/1964 Douglas 91/4373.l56,l60 1 H1964 Meyer et al 92/131 3,165,98l l/l965 Amour 9l/4023,]73337 3/l965 Cunningham 92/l3.5 3,475,000 l0/l969 Fry ct al. 25l/3l X3.608.43l 9/l97l Pease 9l/437 3.626.807 12/197] Shartzer 92/l3.63.657.964 4/]972 Workman ct a]. 9l/l FOREIGN PATENTS OR APPLICATIONS753,866 8/1956 United Kingdom 91/420 Primary Exumr'nerMartin P.Schwadron Assistant Examiner-Abraham Hershkovitz Attorney. Agent, orFirm-Diller, Brown, Ramik & Wight [57] ABSTRACT A fluid operatedactuator for controlling the movement of a member such as a valve andbeing connectable into a fluid circuit including pressure operated meansfor indicating extreme operative positions of the aforementioned member.The actuator including a piston-cylinder assembly, the piston of whichis connectable to the aforementioned member, and pressure relief meansoperative to prevent the pressure acting on the piston reaching apredetermined level which is below that necessary to operate theindicating means. Cut-out means is provided within the piston-cylinderassembly to render the relief means inoperative when the piston reachesor approaches positions corresponding to the aforementioned extremeoperative positions of the associated member, so that the pressureacting on the piston is able to rise sufficiently to operate theindicating means.

27 Claims, 24 Drawing Figures ESE-11160 PATENTEDJUH3 1975 SHEET Q II! 1. mm 5 E EKG s Q swim Q at H mm Q Q m \EQR Q NIB mm m o 2 WW LMSHEET Elli/ll/l/ I'll/Ill FLUID OPERATED ACTUATOR FOR MOVABLE MEMBERSThis application is a division of my copending application Ser. No.73.508. filed Sept. l8, I970, and entitled FLUID OPERATED ACTUATOR FORMOV- ABLE MEMBERS, now US. Pat. No. 3.688.644.

This invention relates to fluid operated actuators for movable members.and is particularly concerned with such actuators for use in actuatorsystems which include means for indicating when the connected memberapproaches or reaches a desired position.

There are numerous movable member installations in which the member iseither hidden or remotely located such that means is required toindicate whether the actuator has moved that member into or towards aparticular position. An example installation requiring such indicatormeans is a valve installation on board a boat or ship, and it will beconvenient to hereinafter describe the invention in relation to such aninstallation.

In view of the remote location of numerous valves on board ships, it isdesirable if not essential to have some means for indicating whethereach valve is in its open or closed position. It is common to controlthe movement of such valves through fluid pressure operated actuators,and the indicator system is usually formed separate to the actuatorsystem although it may also be fluid controlled. The result is a complexand expensive maze of pipe-work, conduit, and ancillary equipment. Inaddition, known indicator systems employing fluid pressure are oftenadversely affected by temperature changes, and may give an incorrectindication if the associated valve is stuck or encounters abnormalresistance between the open and closed positions.

It is a principal object of the present invention to provide a movablemember actuator which is operated by fluid pressure and can beincorporated into a system having means for indicating whether theactuator has reached or is approaching a particular operative condition.It is a further object of the invention to provide such an actuatorwhich is relatively simple in construction and capable of being usedwith indicator means which is not susceptible to normal temperaturevariations. Yet another object of the invention is to provide such anactuator system incorporating indicator means which provides a positiveindication only when the actuator adopts a predetermined operativecondition.

In an actuator system including an actuator according to the invention,the pressurized fluid causing operative movement to the actuator is alsoutilized to operate the indicator means. As a result, an installationincluding such an actuator is much simpler than similar installationsincorporating conventional actuators and indicator means. Basically theactuator includes pressure relief means which is operative to preventthe fluid pressure in at least part of the actuator system reaching apredetermined indicator pressure, and cut-out means for rendering therelief means inoperative when the actuator adopts a particular operativecondition.

When an actuator according to the invention is applied to a marinevalve, it is generally convenient to have the actuator arranged toindicate both the open and closed positions of that valve. The followingparticular embodiments of the actuator will be described as so arranged,but it is to be realized that in some cases indication of a singleposition will be sufficient. Furthermore, it must be understood that theinvention is not restricted to actuation of valves. but is applicable toactuation of almost any other movable member such as doors, gates andhoists, to name but a few.

The following description refers in more detail to these essentialfeatures and further optional features of the invention. To facilitatean understanding of the invention, reference is made to the accompanyingdrawings where these features are illustrated in preferred form. It isto be understood however, that the essential and optional features ofthe invention are not limited to the specific forms of these features asshown in the drawings.

In the drawings:

FIG. I is a partially sectioned side elevational view of an actuatoraccording to the invention, showing the actuator between its two extremeoperative positions;

FIG. 2 is an end view of the actuator shown in FIG. I in the directionof arrow A;

FIG. 3 is an enlarged transverse cross-sectional view taken along lineIII-III of FIG. 1, with parts omitted for convenience of illustration,and showing the control valves as positioned during closing movement ofthe actuator;

FIG. 4 is a cross-sectional view taken along line IV-IV of FIG. 3;

FIG. 5 is a cross-sectional view taken along line V-V of FIG. 3;

FIG. 6 is an enlarged longitudinal cross-sectional view of the pistonassembly of the actuator of FIG. 1, showing that assembly approachingthe position in which a connected valve member is almost closed;

FIG. 7 is a transverse cross-sectional view taken along line VII-VII ofFIG. 6;

FIG. 8 is a sectional view taken along line VIlIVllI of FIG. 6;

FIG. 9 is a view similar to FIG. 6 but showing the piston assemblylocation when the connected valve member is closed;

FIG. 10 is a view similar to part of FIG. 1, but showing the pistonassembly location when return movement of the assembly is initiated tocause opening movement of the connected valve member;

FIG. 11 is a view similar to FIG. 3 but showing the control valves aspositioned during opening movement of the actuator;

FIG. 12 is a view similar to FIG. 6 but showing the piston assembly aslocated when the connected valve member is approaching its fully openposition;

FIG. 13 is a view similar to FIG. 9 but showing the piston assemblylocation when the connected valve member is fully opened;

FIG. 14 is a view similar to FIG. 3 but showing the control valves inthe closed position as adopted when the actuator is inoperative;

FIG. 15 is an enlarged cross-sectional view taken along line XVXV ofFIG. 2',

FIG. 16 is an enlarged cross-sectional view taken along line XVI-XVI ofFIG. 2;

FIG. 17 is a diagrammaticwiew of an actuator according to FIG. Iconnected 'nto a fluid circuit;

FIG. 18 is a side elevational view of an alternative embodiment of anactuator according to the invention;

FIG. 19 is a side elevational view of the actuator shown in FIG. 18taken in the direction of arrow B, and partially sectioned forconvenience of illustration;

FIG. 20 is a cross-sectional view taken along line XX-XX of FIG. 18;

FIG. 21 is a cross-sectional view taken along line XXI-XXI of FIG. 19,parts being omitted for convenience of illustration;

FIG. 22 is a cross-sectional view taken along line XXII-XXII of FIG. 19;

FIG. 23 is a cross-sectional view taken along line XXIIIXXIII of FIG.19; and

FIG. 24 is a cross-sectional view taken along line XXIV-XXIV of FIG. 23.

FIGS. 1 to 16 of the drawings show one particular embodiment of theinvention for use with slidable gate or other linear movement valves, inwhich the actuator includes a primary piston 2 and a secondary piston 3,both of which are mounted within a single cylinder 4 for axial movementrelative thereto. The primary piston 2 divides the cylinder 4transversely into two chambers 5 and 6, and for convenience thosechambers will be hereinafter called opening and closing chambersrespectively. A piston rod 7 is secured to the secondary piston 3 formovement therewith and extends through end wall 8 of the cylinder 4 forconnection to a valve member 9 (see FIG. 17). and it is preferred thatthe opening chamber 5 is defined between the end wall 8 and the primarypiston 2.

The end wall 8 may form part of an end cap 11 secured to the cylinderbody 12 and having a fluid passageway 13 through a barrel portion 14thereof and connecting at one end with the opening chamber 5.Alternatively however, the fluid passageway 13 may be formed through theside wall of the cylinder body 12. A conduit 15 may be connected to thefluid passageway 13 and connected into a fluid circuit of the actuatoras hereinafter described. The bore of the cap 11 through which thepiston rod 7 passes is preferably defined in part by a removable bush 16having any suitable means 17 therein to form a substantially fluid-tightseal with the piston rod 7 whilst allowing axial movement of that rod.

A cap member 18 may also close the opposite end of the cylinder 4, andthat member preferably has a control valve block 19 formed integraltherewith (see FIGS. 2 to 5). although the block 19 may be formed as aseparate element and connected to any other appropriate part of theactuator as desired. In the example arrangement shown, the valve block19 includes two valve compartments 21 and 22, each of which contains aspring influenced one-way valve 23. The compartment 21 .is in directcommunication with the cylinder chamber 6 through a passage 24, andcompartment 22 communicates with the conduit 15 through a passage 25 andis thereby connected with the cylinder chamber 5. It will be appreciatedthat the external conduit 15 may be substituted by some other means suchas a passage formed within the wall of the cylinder body 12.

An intermediate compartment 26 may be formed within the block 19 andcontains an axially slidable spool element 27 arranged to open eithervalve 23 as hereinafter described. Ports 28 and 29 communicate with thecompartment 26 on opposite sides respectively of the spool element 27 asshown in FIG. 3, and each serves as inlet and outlet means for arespective one of the chambers 5 and 6. When the actuator is connectedinto a fluid system, a supply-exhaust conduit 31 is connected to port28, and a similar conduit 32 is connected to the port 29. The spoolelement 27 serves as a fluid seal between the ports 28 and 29.

Each valve 23 preferably includes a conical nose section 33 whichnormally projects into and closes a valve orifice 34. The spring settingof each valve 23 is such that the valve may be opened by the normaloperating pressure of the fluid used to operate the actuator. A stem 35extends axially from each of the two opposite ends of the spool element27 to engage and open a respective valve 23 as hereinafter described.For convenience of description, identifying reference numerals of thevalves 23 and associated parts connected to chambers 5 and 6 will behereinafter suffixed "a and b" respectively.

The primary piston 2 may be axially movable relative to the secondarypiston 3 and the piston rod 7 between two limit stops each of which ispreferably secured to the secondary piston 3. [n the arrangement shownin FIGS. 1 and 6, one such stop is formed by an annular nut 36 securedto the terminal end portion of the secondary piston 3, and the otherstop is formed by a collar 37 secured to the secondary piston 3 adjacentthe piston rod 7. It is convenient to form the collar 37 integral withthe secondary piston 3. Although the secondary piston 3 is shownintegral with the piston rod 7, it will be appreciated that those twoparts may be formed separate and attached by a screw thread or otherconnection.

As shown particularly in FIG. 6, the primary piston 2 includes an innersleeve formed of two parts 38 and 39 which are slidable on the secondarypiston 3 between nut 36 and collar 37, and an outer sleeve 41 which isslidable on the inner sleeve parts 38 and 39. The inner sleeve is madein two parts as a matter of convenience for assembly with the outersleeve 41, and each part 38 and 39 has an enlarged end portion 42 and 43respectively which defines an annular shoulder 44 and 45 respectively.The two shoulders 44 and 45 are opposed and each provides a combinedstop and sealing surface for cooperation with the outer sleeve 41 ashereinafter described.

The pressure relief means preferably includes a pair of relief valves 46and 47 which are arranged to prevent the fluid pressure within thecylinder chambers 5 and 6 respectively from reaching the aforementionedindicator pressure that is, when the relief means is not renderedinoperative by the cut-out means. Although the relief valves 46 and 47may be located externally of the cylinder 4, it is preferred to locatethem within a respective relief passage 48 and 49 formed within thesecondary piston 3. Each relief passage 48 and 49 may extend axially ofthe piston rod 7, and have laterally extending entrance portions 51 and52 respectively through which communication is made with the respectivechambers 5 and 6. The valve 46 exhausts to chamber 6 through a passage53, and valve 47 exhausts to chamber 5 through a passage 54. Obviously,an arrangement may exist in which the two relief valves 46 and 47 arelocated within a single relief passage.

Each of the relief valves 46 and 47 may include a valve member 55movable axially relative to the associated relief passage, and acooperable valve seat 56. In one form as shown in FIG. 6, each valvemember 55 is a ball which is locatable within an orifice 56 defining itsrespective valve seat, and the valve member is preferably biased by aspring 57 towards that seat. Each spring 57 acts on its respective valveball 55 through a slide member 58 which is provided with flat surfaces59 (see FIG. 8) or other clearance means or apertures to allow passageof fluid thereby.

The seat 56 of valve 47 may be formed by the bore of a bush 61 mountedwithin the secondary piston 3, and the axial position of the bush 6] ispreferably adjustable to enable variation of the compression of valvespring 57 and consequently the valve closing force. The seat 56 of valve46 may be formed by one end of the relief passage 48, and the passage 53is preferably formed as the bore of a bush 62 adjustably mounted in thesecondary piston 3 to enable variation of the compression of the spring57 for the valve 46.

The pressure relief means may also include a pair of passages 63 and 64formed through respective parts 38 and 39 of the primary piston innersleeve, and they are located to communicate with the lateral passages 52and 51 respectively. That communication may be maintained over thelength of movement of the primary piston 2 relative to the secondarypiston 3, although internal grooves 65 and 66 of the inner sleeve parts38 and 39 respectively (see FIG. 7).

Cut-out means for the arrangement so far described preferably includes apair of abutments 67 and 68 (see FIG. 1) which limit axial movement ofthe primary piston 2 with the secondary piston 3. The abutments 67 and68 are located adjacent respective opposite ends of the cylinder body12, and each is arranged to stop movement of the primary piston 2towards that end whilst allowing continued movement of the secondarypiston 3. In one form, the abutment 67 is defined by the axially innerend face of the barrel portion 14 of the end cap 11, and abutment 68 isdefined by the axially inner end face of a barrel portion 69 of the endcap 18. The secondary piston is receivable within the bore of each ofthe barrel portions 14 and 69 to allow the aforementioned continuedmovement. Obviously, any other suitable abutment may be used such as asimple pin projecting radially inwardly from the side wall of thecylinder 4, or the cross-sectional area of the cylinder body 12 may bereduced to form appropriate stop shoulders.

Safety means is preferably provided within the valve block 19 to preventthe pressure within either chamber 5 or 6 reaching a predetermined uppersafe limit. In this regard, the control valves 23a and 23b remain closedwhen the actuator is not in operation, as shown in FIG. 14, andconsequently fluid is trapped in both chambers 5 and 6. That fluid mayexpand with a rise in ambient temperature, and the resulting increase inpressure may cause damage to the actuator if not relieved. One suchsafety means is two spring influenced one-way valves 71 and 72 (seeFIGS. and 16) which communicate with the chambers 5 and 6 respectively.

Safety valve 71 includes a valve element 73 contained within a chamber74 and biased to normally close the orifice 75. The orifice 75communicates direct with the port 25 through a passage 76 to allow fluidunder pressure to be exhausted from the chamber 5 by way of conduit 15.The valve chamber 74 communicates with the port 29 through a passage 77,whereby fluid exhausted through the valve 71 is able to drain to a tankor reservoir by way of conduit 32.

The safety valve 72 also comprises a valve element 78 contained within achamber 79 and biased to normally close an orifice 81. The orifice 81communicates direct with the passage 24 through a passage 82 to allowfluid under pressure to be exhausted from the chamber 6 by way of thepassage 24. The valve chamber 79 communicates with the port 28 through apassage 83, whereby fluid exhausted through the valve 72 is able todrain to a tank or reservoir by way of the conduit 31.

In an example installation including the previously described actuatoras shown diagrammatically in FIG. 17, the end portion of the piston rod7 located outside the cylinder 4 is secured to the slide 9 of the gatevalve 84, and means is preferably provided whereby the axial position ofthe slide 9 may be adjusted relative to the piston rod 7.

One or both of the primary piston abutments 67 and 68 may be adjustablein the axial direction of the cylinder 4 to enable variation of the stopposition or positions of the piston 2. Conduits 31 and 32 areselectively connectable to a source of pressurized fluid through adirectional control valve 85. In one position of the valve 85, conduit31 serves to supply fluid to chamber 5, and conduit 32 functions as adrainpipe for chamber 6. In another position of the valve 85, conduit 32feeds chamber 6, and conduit 31 drains chamber 5. Alternatively, aseparate feed and drainpipe may be connected to each chamber andcontrolled by appropriate valving as required.

A pressure sensitive switch 86 is preferably connected to each of theconduits 31 and 32, and each of the switches 86 may be operativelyconnected to a suitable indicator device (not shown). Each indicatordevice may be arranged to produce a visual and/or audible signal, but itis generally convenient to employ simple incandescent lamps as theindicator devices. Chokes 87 and an accumulator 88 may be provided inthe circuit as required according to known procedures and techniques.

The actuator may be hydraulically or pneumatically operated, buthydraulic operation is generally preferred.

Assuming the gate valve 84 is in an open position, the slide 9 may bemoved towards the closed position by causing pressurized fluid to enterthe actuator circuit through conduit 32 which feeds that fluid into thecylinder chamber 6 through port 29. Fluid passing through port 29 entersthe intermediate compartment 26, and the pressure thereof opens thevalve 23b (FIG. 3) so that the fluid is then able to pass from valvecompartment 21 to the chamber 6 through passage 24. That same fluidpressure urges the spool element 27 to open the valve 230 by engagementbetween the stem 35a and nose section 330 as shown in FIG. 3, andconsequently chamber 5 is able to exhaust through conduit 15 to port 28and conduit 31. In normal operation, that fluid pressure is insufficientto open the relief valve 47 so that the fluid functions to urge theprimary and secondary pistons 2 and 3 towards the end cap 11 of thecylinder 4. When the fluid is first introduced into the chamber 6however, the primary piston 2 moves relative to the secondary piston 3away from the end cap 18 and subsequently engages the stop collar 37(see FIG. 1), after which the two pistons 2 and 3 move together with thepiston rod 7 thereby progressively closing the gate valve 84.

During such closing movement, the relief passage 49 is in communicationwith the chamber 6 through the lateral entrance 52 and the passage 63 asshown in FIGS. 1 and 6. If the gate valve slide 9 sticks or encountersan obstacle such as to increase the resistance to further closingmovement, or prevent such further movement, a build-up of pressure willnaturally occur within the chamber 6. The relief valve 47 is arranged toopen before that pressure reaches the aforementioned indicator pressurewhich is effective to operate the pressure sensitive switch 86 connectedto the conduit 32. Thus. fluid is able to escape from the chamber 6through the passages 49 and 54 to the chamber 5, from where it isdischarged through the conduits l and 31.

Assuming closing movement of the actuator continues, a condition iseventually achieved in which the outer sleeve 41 of the primary piston 2engages and is stopped by the abutment 67 as shown in FIG. 6. The innersleeve parts 38 and 39 and secondary piston 3 are able to continuemoving together towards the end cap 11, until the shoulder 44 engagesthe adjacent end of the sleeve 41 as shown in FIG. 9. In this position,communication between chamber 6 and the relief valve 47 is closed, andthe pressure within the chamber 6 acts on the secondary piston 3 aloneso that the piston rod 7 continues to move in the closing direction andthe stop collar 37 is progressively moved into the bore of the barrelportion 14. Thus, when the valve slide 9 closes, pressure is built upwithin the chamber 6, and consequently the conduit 32, beyond theindicator pressure thereby resulting in operation of the associatedindicator device.

When it is desired to open the gate valve 84, the directional controlvalve 85 is operated so that pressure fluid is introduced to the chamber5 through the conduit 31, and the conduit 32 connects the chamber 6 todrain. It will be appreciated that the closing force of the actuatorduring the last part of the closing movement is dependent upon the axialend area of the secondary piston 3, since both sleeves of the primarypiston 2 are held by the abutment 67, and that area is preferablysubstantially less than the combined end face area of the primary andsecondary pistons which is subjected to fluid pressure within thechamber 5. Thus, for the same fluid pressure. the opening force of theactuator is greater than the final closing force thereby ensuringeffective operation of the actuator.

When pressurized fluid is fed into the valve block compartment 26 by wayof conduit 31 and port 28, valve 23a opens as shown in FIG. 11 to allowthe fluid to enter port 25 from where it passes to the chamber 5 by wayof the conduit 15. The fluid pressure in compartment 26 also moves thespool element 27 into a position such that the stem 35b engages thevalve nose section 33b and causes the valve 23b to open as shown in FIG.11, and thereby allow the chamber 6 to exhaust through passage 24 to theport 29 and connected conduit 32.

Initial entry of pressurized fluid into the chamber 5 causes the primarypiston 2 to adopt the position shown in FIG. in which the inner sleevepart 38 engages the nut 36 and the outer sleeve 41 engages the shoulder44. After that position has been reached, the primary and secondarypistons move together towards the end cap 18 thereby causing theconnected valve 84 to open. Also in this position of the pistons 2 and3, the relief passage 48 is in communication with the chamber 5 throughthe lateral entrance 5] and the passage 64 as shown in FIGS. 10 and 12.

During opening movement, pressure build-up within the chamber 5 isrelieved in the same manner as described in relation to the chamber 6during the closing movement, except that the pressure relief is effectedthrough opening of the relief valve 46 with consequent discharge offluid to the chamber 6 through the passage 53. Fluid escapes from thechamber 6 through the passage 24 and port 29 to the conduit 32 which isconnected to drain during this particular operation, as previouslydescribed. It will be appreciated that under normal conditions. thepressure in the chamber 5 will not rise to the relief pressure unlessabnormal resistance to the opening movement of the valve slide 9 isencountered.

As the valve slide 9 approaches the fully opened position, the outersleeve 41 of the primary piston 2 engages the abutment 68 as shown inFIG. 12. and is thereby held against further movement in the openingdirection. The inner sleeve parts 38 and 39 and secondary piston 3however, continue to move in that direction as fluid pressure within thechamber 5 acts against their combined effective end area. The shoulder45 of the inner sleeve part 39 is thereby moved into engagement with theadjacent end of the sleeve 41 as shown in FIG. 13, after which thesecondary piston 3 continues to move alone and is progressively moveddeeper into the bore of the barrel section 69. Engagement between theouter sleeve 41 and the abutment 68 results in closing of communicationbetween the entrance 5] of the relief passage 48 and the chamber, andthat oc curs as the valve slide 9 arrives at or approaches its fullyopened position. Thus, build-up of pressure in the chamber 5 is causedby cessation of movement of the piston rod 7 and fluid cannot escapethrough the relief passage 48 so that the actuator device associatedwith the conduit 31 is subsequently actuated.

When pressurized fluid is not being fed through either of the conduits31 or 32 i.e., the actuator is static the control valves 23a and 23bassume closed positions as shown in FIG. 14. Thus, fluid is trapped inboth of the chambers 5 and 6 and the escape means previously describedenables relief of pressure from those chambers if the pressure thereinbecomes excessive because of fluid expanding under increasedtemperature. If the pressure increase occurs in chamber 5, that increasewill be also present in the conduit 15 because of its constantconnection with the chamber 5, and the conduit 15 is also in constantcommunication with safety valve 71 through port 25 and passage 76 asshown in FIG. 15. The valve 71 is arranged to open when the pressurereaches a predetermined limit, and thereby allows exhaust of fluid toconduit 32 by way of passage 77 and port 29.

If the pressure increase occurs in the chamber 6, safety valve 72 willopen because it is in constant communication with the chamber 6 throughinterconnecting passages 24 and 82. Exhaust of fluid from the valvechamber 79 to conduit 31, occurs through passage 83 and port 28 as shownin FIG. 16.

The construction described may be modified in numerous ways to achievethe basic requirements of the invention. For example, a single reliefvalve may serve both chambers 5 and 6 of the cylinder 4 if theassociated relief passages are appropriately arranged. Also, theactuator may be constructed to provide a positive indication in oneposition only, in which case the relief means and cut-out means aresuitably modified. Still further, the stop collar 37 may be containedwithin or enveloped by the primary piston 2 so as to be capable of onlylimited axial movement relative thereto. Also, the primary piston 2 mayconsist of a single sleeve section. in which case the relief meanswithin the piston assembly is arranged to suit.

Yet another embodiment of the invention is shown in FIGS. 18 to 24, andthat embodiment is particularly suitable for use with a rotary orsemi-rotary valve such as a butterfly valve. For convenience, componentsof this embodiment which correspond to components of the first describedembodiment will be identified by like reference numerals, except thatthose numerals will form part of the series 101 to 200.

According to this embodiment, the cylinder 104 contains a pistonassembly 101 having a pair of cylindrical end portions 189 which arejoined by an intermediate portion 191 as shown in FIG. 20. Each endportion 189 forms a movable end wall of a respective one of the cylinderchambers 105 and 106, and end caps 111 and 118 are secured to oppositeends of the cylinder 104 substantially as in the previous embodiment. Itis preferred in this embodiment however, that a separate valve block 119be formed integral with or attached to each of the end caps 111 and 118,and each of those blocks 119 contains a control valve 123 as shown inFIG. 21, which is operative to control passage of fluid into and out ofthe adjacent cylinder chamber. Control valves 123a and l23b areassociated with chambers 105 and 106 respectively.

Each control valve 123 preferably includes the same basic components asthe valves 23 described in relation to the first embodiment, andfunctions in the same manner. The principal advantages are that eachspool element 127 includes a single stem 135, and the compartments 126communicate through two conduits 192 and 193 as shown in FIGS. 18 and21. The conduit 192 connects the front side of compartment 126a to therear side of compartment 126!) so that pressurized fluid entering thecompartment 1260 from the conduit 131 and through port 128 is effectiveto cause valve 123b to open through the medium of the spool element12721 and thereby allow the chamber 106 to exhaust through port 129 toconduit 132. The conduit 193 connects the front side of the compartment126b to the rear side of the compartment 126a to enable both valves 123band 1230 to open when the conduits 132 and 131 function as feed anddrain lines respectively.

The valve compartment 122 of valve 123a communicates with the chamber105 through a passage 125 as shown in FIGS. 2], 23 and 24, and a passage124 connects the compartment 121 of the valve 123!) to the chamber 106as also shown in FIG. 21. FIG. 21 shows the valves 123a and 123kpositioned as when fluid is fed to the chamber 105 through the conduit131 and is drained from the chamber 106 through the conduit 132.

Relief means is preferably located externally of the cylinder 104 and inthe particular arrangement shown includes a valve block 194 containingrelief valves 146 and 147 which function substantially the same as thecorresponding valves 46 and 47 of the previously described embodiment.Relief valve 146 includes a member 155 arranged to close an orifice 156of a relief passage 148 which communicates with the chamber 105 througha passage 150 of the block 194 and interconnected passages 151 and 164of the adjacent block 119, as shown in FIG. 23. The valve 146 exhauststo the chamber 106 through a passage 153 and connecting port 195 ofthevalve block 194, and a conduit [96 (see FIG. 19) which connects the port195 to interconnected passages 152 and 163 of the other valve block 119(i.e., that containing valve 12317) as shown in FIG. 19. The passages152 and 163 communicate direct to the chamber 106 and correspond topassages 151 and 164 respectively of the other block 119. In thisregard, the passage arrangement in both of the valve blocks 119 issubstantially identical.

The relief valve 147 receives high pressure fluid from the chamber 106through conduit 196, port 195 and a passage 149 of the valve block 194.It is exhausted to the chamber through the interconnected passages 150,151 and 164.

A manually operable by-pass valve 197 is preferably connected to therelief valve assembly to enable the valves 146 and 147 to be bypassedwhen desired. For example, such by-pass allows manual movement of thepiston assembly 101 during maintenance of the actuator. The valve 197includes a closure member 198 which is operable by a hand wheel 199 toopen and close a by-pass orifice 201 which extends between and connectsthe port and passage 148 as shown in FIG. 22.

Cut-out means for this embodiment preferably includes a pair of stopelements 202 and 203 which contain the passages 164 and 163respectively. The axially inner end face of the element 202 forms theabutment 167 and the corresponding end face of the element 203 forms theabutment 168. Both the elements 202 and 203 may be adjustably mountedwithin the respective valve blocks 119 as shown in FIG. 19, to allowvariation of the axial position of the respective end faces 167 and 168,and consequently the two stop positions of the pis ton assembly 101.

It will be appreciated from the foregoing that the relief valve 146 isrendered ineffective when an end portion 189 of the piston assembly 101engages the abutment face 168 of the stop element 203. In that positionof the piston assembly 101, the passage 163 is closed and high pressurefluid is unable to escape from the chamber 105 into chamber 106, eventhough the relief valve 146 may be open and connects the conduit 196 tothe chamber 105. Thus, under such conditions, the pressure withinchamber 105, and consequently conduit 131, progressively increases untilthe indicator pressure is reached. During operation of the actuatorbetween the stop positions of the piston assembly, the chamber 106remains in communication with the relief valve assembly through thepassage 163, but as with the previously described embodiment, the reliefvalve 146 is arranged so as to remain closed under normal operatingpressure conditions.

The abutment face 167 and relief valve 147 function in the same manneras described for abutment face 168 and relief valve 146, whenpressurized fluid is introduced to the chamber 106 for moving the pistonassembly 101 in the opening direction.

Safety means similar to that described in relation to the firstembodiment is also preferably provided in each valve block 119. Forconvenience, only the safety means in the block 119 containing the valve1230 will be described, and it is to be understood that identical meansis provided in the other block 119.

Referring to FIGS. 23 and 24, a safety valve 171 includes a valve member173 contained with a compartment 174 and spring influenced to normallyclose an orifice 175. The orifice 175 communicates direct with thechamber 105 through passage I25, and the compartment 174 communicateswith the port 128 through a passage 177. Thus, when valves 123a and l23bare closed, excessive pressure within the chamber 105 causes the valve171 to open and exhaust fluid to the conduit 131 through the port 128.The corresponding safety valve of the block 119 containing control valve123b, functions in the same way to exhaust chamber 106 to the conduit132.

In the arrangement particularly shown in the drawings, the pistonassembly I01 is connectable to the valve element ofa butterfly orsimilar semirotary valve, and that connection may be effected through arack and pinion. The rack 204 may be formed on the intermediate portion191 of the piston assembly 101 as shown in FIG. 20, and the pinion 205is secured to a rotatable drive shaft 206 adapted to be operativelyconnected to the butterfly valve. The shaft 206 may be rotatably mountedin a housing 209 formed integral with or connected to the cylinder 104.

A visual indicator may be associated with the actuator in addition to anindicator device of the kind described in relation to the previousembodiment. Such an additional indicator may include a pointer 207secured to the shaft 206, and a scale 208 secured to the cylinder 104,as shown in FIG. 19. ln FIG. 19 the indicator shows the piston assembly101 to be in its intermediate position in which a connected valve memberis located between its open and closed positions.

It will be clear that an actuator construction according to FIGS. 18 to24, may be connected into a fluid system to operate in basically thesame manner as the construction first described. That is, in operationthe actuator of FIGS. 18 to 24 may be included in a fluid circuit asshown in FIG. 17. If desired, the two piston end portions 189 may be ofa different size to produce a difference in the closing and openingforces for the same fluid pressure.

The basic actuator construction last described may be also applied tolinear movement valves, but for such an application it may be moreconvenient to have each piston end portion 189 contained withinrespective spaced cylinders rather than a single cylinder as shown anddescribed. Any of the relief and cut-out means described may be usedwith such a construction, and the connection of each piston end portion189 to the valve slide may take any convenient form.

Although the specific embodiments of the actuator described haveincluded a piston or pistons as the force transmitting element of theactuator, it is to be understood that other types of elements may beused. For example, the force transmitting element may be a diaphragm,bellows, or any other element adapted to translate fluid pressure intomechanical movement.

It will be appreciated that an actuator as described has considerablepotential because of its simple yet effective construction. In a marineapplication as described for example, the expense and complexity of theactuator and indicator installations are both significantly reduced.Furthermore, the actuator is not susceptible to giving false indicationsbecause of leakage in the fluid system, and it is substantiallyfail'safe in that all valves return to a closed position if the fluidsystem fails.

Finally, it is to be understood that various alterations, modificationsand/or additions may be introduced into the preferred constructions andarrangements of parts previously described without departing from thespirit or ambit of the invention.

Having now described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. An actuator system including: a fluid operated actuator connectableto a member and being operable to cause movement of said member; saidactuator including a cylinder and a force transmitting element which ismovable axially within said cylinder under the influence of fluidpressure and is operatively connected to said member to be actuated;said force transmitting element dividing the interior of said cylindertransversely into two chambers; said cylinder having means operativelyconnected to both of said chambers whereby pressurized fluid may beintroduced to or removed from either of said chambers through said meansaccording to the desired direction of movement of the force transmittingelement; a source of pressurized fluid; control means operable toconnect said fluid source to a fluid circuit of said actuator to causeoperation of the actuator; relief means connected into said fluidcircuit and being operable to prevent the fluid pressure acting on saidactuator from reaching a predetermined level; cut-off means operable torender the relief means inoperative when said actuator adopts aparticular operative position but when in said operative position stillpermitting pressurized fluid to be introduced or exhausted from eithercylinder chamber through said means connected to both of said chambersto enable continued operation of said actuator and movement of saidforce transmitting element and member; and indicator means connected tosaid fluid circuit so as to be operated when the pressure acting on saidactuator reaches said predetermined pressure, and thereby indicates theoperative position of said actuator and consequently the position of thesaid member connected thereto.

2. An actuator system according to claim I, wherein said control meansincludes a directional control valve which is selectively operable tocause said actuator to move said member in either of two oppositedirections.

3. An actuator system according to claim 1, wherein said indicator meansincludes a signal device and a pressure sensitive switch operativelyconnected to both said signal device and said fluid circuit, wherebysaid switch is actuated to energize said signal device when the fluidpressure acting on said actuator approaches said predetermined level.

4. An actuator system according to claim 1, wherein said member is thevalve element of a slide gate valve.

5. An actuator system according to claim 1, wherein said member is thevalve element of a semi-rotary butterfly valve.

6. An actuator system according to claim I, wherein said actuatorincludes a cylinder and a piston assembly which is movable axiallywithin said cylinder under the influence of fluid pressure and isconnectable to said member to be actuated, said pressure relief meansbeing operable to prevent the pressure acting on said piston assemblyreaching said predetermined level, said piston assembly divides theinterior of said cylinder transversely into two chambers, and saidpressure relief means communicates with at least one of said chamberswhen said cut-off means is inoperative.

7. An actuator system according to claim 6, wherein said cylinderincludes inlet means for both said chambers whereby pressurized fluidmay be introduced to either of said chambers according to the desireddirection of movement of the piston assembly; and said pressure reliefmeans is operable to relieve pressure from whichever of said chambers issubjected to said pressurized fluid, so long as said cut-off meansremains inoperative.

8. An actuator system according to claim 7, wherein said cut-off meansincludes at least part of said piston assembly which, in said particularoperative position, is located to close communication between one ofsaid chambers and said relief means.

9. An actuator according to claim 8, wherein said piston assembly has afirst said particular operative position in which said part thereofcloses communication between one of said chambers and said pressurerelief means, and is positioned adjacent the end of the cylinder remotefrom that chamber; and a second said particular operative position inwhich said part thereof closes communication between the other saidchamber, and is positioned adjacent the end of said cylinder oppositesaid firstmentioned end thereof.

10. An actuator system according to claim 9, wherein means is providedto reduce the effective end surface area of the piston assembly to whichpressurized fluid is applied to cause movement of the piston assembly inone direction, said means being arranged to cause said area reductionwhen said piston assembly is at or adjacent a said particular operativeposition; and the opposite effective end surface area of the pistonassembly to which pressurized fluid is applied for initially moving thepiston assembly in the opposite direction, is greater than said reducedarea.

11. An actuator system according to claim 7, wherein a single portdefines both the inlet means and the outlet means for each said chamber,said ports communicating with their respective chambers through apressure operated control valve arranged to open both said ports totheir respective chambers in response to pressurized fluid being fedinto either one of said ports, whereby the port receiving saidpressurized fluid functions as inlet means for its respective chamberand the other said port functions as outlet means for its chamber.

12. An actuator according to claim 11, wherein said control valve isbiased to normally close communication between both said ports and theirrespective chambers, and safety valve means is connected to both saidchambers to prevent the pressure therein exceeding a predetermined upperlimit.

13. An actuator system according to claim 6, wherein said pistonassembly includes a secondary piston which is formed by or attached toan end portion of a piston rod which forms at least part of saidconnecting means, and a primary piston which is slidably mounted on saidsecondary piston for limited axial movement relative thereto; saidpiston rod projecting through an end wall of said cylinder and beingmovable axially relative to the cylinder with said secondary piston;said pressure relief means includes a passage formed through saidsecondary piston to provide communication between said chambers; andsaid cut-off means includes at least part of said primary piston whichis arranged to open or close said communication according to its axialposition relative to said secondary piston.

14. An actuator system according to claim 13, wherein; said primarypiston includes an inner sleeve section slidable on said secondarypiston, and an outer sleeve section which is slidable on said innersleeve section; stop means is provided on said secondary piston to limitaxial movement of said inner sleeve section relative to the secondarypiston; and said outer sleeve section is located between opposed annularshoulders of said inner sleeve section and is engageable therewith tolimit its axial movement relative to said inner sleeve section.

15. An actuator system according to claim 14, wherein an abutment isprovided within said cylinder for engagement by said outer sleevesection to limit axial movement of said outer sleeve section relative tothe cylinder in one direction; and said inner sleeve section andsecondary piston are able to move further in said one direction aftersaid outer sleeve section and said abutment engage, whereby said outersleeve section is located relative to said secondary piston to closesaid relief passage.

16. An actuator system according to claim 15, wherein said reliefpassage is closed when said outer sleeve section engages both the saidabutment and the said annular shoulder located remote from saidabutment; and in that position of the piston assembly the effective endarea thereof which is engageable by pressurized fluid to move the saidsecondary piston further in said one direction, is smaller than theeffective opposite end area of the piston assembly which is engageableby pressurized fluid to move said assembly in the direction opposite tosaid one direction.

17. An actuator system according to claim 16, wherein two said passagesare formed through said secondary piston, and said primary piston partis movable axially relative to the secondary piston between twopositions in each of which said part closes a respective said passage.

18. An actuator system according to claim 17, wherein said outer sleevesection engages a respective said abutment in each of said twopositions, and a reduction in the said effective end area of the pistonas sembly occurs in each of said positions, the said end area reducedbeing opposite for each of said positions.

19. An actuator system according to claim 6, wherein said pistonassembly has a first said particular operative position in which a firstpart of said piston assembly closes communication between one of saidchambers and said pressure relief means, and is positioned adjacent theend of the cylinder remote from that chamber; and a second saidparticular operative position in which a second part of said pistonassembly closes communication between the other said chamber and saidpressure relief means, and is positioned adjacent the end of saidcylinder opposite said firstmentioned end thereof.

20. An actuator system according to claim 6, wherein; said pistonassembly includes two cylindrical end portions, an intermediate portionjoining said end portions, and a rack formed on said intermediateportion and drivably engageable with a pinion projecting through anaperture in a side wall of said cylinder, said rack and pinion formingat least part of said connecting means; said relief means includes amain relief passage providing a communication between said chambers; andsaid cut-out means includes at least one of said end portions of thepiston assembly which is arranged to open or close said communicationbetween said cham bers according to the axial position of the pistonassembly relative to said cylinder.

21. An actuator system according to claim 20, wherein said main reliefpassage is defined by a conduit located externally of said cylinder andwhich communicates with each said chamber through a respective passageextending axially of the cylinder and arranged to be closed by theadjacent end surface of the piston assembly.

22. An actuator system according to claim 21, wherein each said axialpassage is formed in a stop element which projects axially into saidcylinder from a respective end wall thereof, the end surface of eachstop element which is located within said cylinder serving as anabutment for the adjacent end of said piston assembly and having therespective said axial passage opening therethrough.

23. An actuator system according to claim 22, wherein each said stopelement is adjustably mounted in the respective cylinder end wall toenable variation of the axial position of the said end surface thereof.

24. An actuator system according to claim 20, wherein said relief meansinclude relief valve means connected to said main relief passage andoperable to open and close communication between said chambers throughsaid main passage, according to the fluid pressure existing in saidchambers.

25. An actuator system according to claim 24, wherein said relief valvemeans includes a valve housing having a compartment therein, said mainpassage communicating direct with said compartment and with one of saidcylinder chambers, and compartment inlet and outlet passages each ofwhich communicates with the other said cylinder chamber and iscontrolled by a respective one-way valve, the two said one-way valvesbeing arranged to respectively allow fluid into and out of saidcompartment.

26. An actuator system according to claim 20, wherein said of theopposite end walls of said cylinder is defined by a valve block whichincludes: a combined fluid supply and exhaust port, a pressure operableoneway valve which is normally biased into a closed position andcontrols fluid flow through said port, and an axially slidable spoolelement which is pressure operable to open said one-way valve; andrespective drain passage means connects the said port of each said valveblock to a rear side of the said spool element of the other said valveblock, whereby pressurized fluid supplied to one said chamber throughthe adjacent said valve block is operative to cause the said spoolelement of the other said valve block to open its respective saidone-way valve and thereby allow said other chamber to exhaust.

27. An actuator system according to claim 26, wherein safety valve meansis provided within each said valve block and communicates with theadjacent said chamber to prevent the pressure therein exceeding apredetermined upper limit.

1. An actuator system including: a fluid operated actuator connectableto a member and being operable to cause movement of said mEmber; saidactuator including a cylinder and a force transmitting element which ismovable axially within said cylinder under the influence of fluidpressure and is operatively connected to said member to be actuated;said force transmitting element dividing the interior of said cylindertransversely into two chambers; said cylinder having means operativelyconnected to both of said chambers whereby pressurized fluid may beintroduced to or removed from either of said chambers through said meansaccording to the desired direction of movement of the force transmittingelement; a source of pressurized fluid; control means operable toconnect said fluid source to a fluid circuit of said actuator to causeoperation of the actuator; relief means connected into said fluidcircuit and being operable to prevent the fluid pressure acting on saidactuator from reaching a predetermined level; cut-off means operable torender the relief means inoperative when said actuator adopts aparticular operative position but when in said operative position stillpermitting pressurized fluid to be introduced or exhausted from eithercylinder chamber through said means connected to both of said chambersto enable continued operation of said actuator and movement of saidforce transmitting element and member; and indicator means connected tosaid fluid circuit so as to be operated when the pressure acting on saidactuator reaches said predetermined pressure, and thereby indicates theoperative position of said actuator and consequently the position of thesaid member connected thereto.
 1. An actuator system including: a fluidoperated actuator connectable to a member and being operable to causemovement of said mEmber; said actuator including a cylinder and a forcetransmitting element which is movable axially within said cylinder underthe influence of fluid pressure and is operatively connected to saidmember to be actuated; said force transmitting element dividing theinterior of said cylinder transversely into two chambers; said cylinderhaving means operatively connected to both of said chambers wherebypressurized fluid may be introduced to or removed from either of saidchambers through said means according to the desired direction ofmovement of the force transmitting element; a source of pressurizedfluid; control means operable to connect said fluid source to a fluidcircuit of said actuator to cause operation of the actuator; reliefmeans connected into said fluid circuit and being operable to preventthe fluid pressure acting on said actuator from reaching a predeterminedlevel; cut-off means operable to render the relief means inoperativewhen said actuator adopts a particular operative position but when insaid operative position still permitting pressurized fluid to beintroduced or exhausted from either cylinder chamber through said meansconnected to both of said chambers to enable continued operation of saidactuator and movement of said force transmitting element and member; andindicator means connected to said fluid circuit so as to be operatedwhen the pressure acting on said actuator reaches said predeterminedpressure, and thereby indicates the operative position of said actuatorand consequently the position of the said member connected thereto. 2.An actuator system according to claim 1, wherein said control meansincludes a directional control valve which is selectively operable tocause said actuator to move said member in either of two oppositedirections.
 3. An actuator system according to claim 1, wherein saidindicator means includes a signal device and a pressure sensitive switchoperatively connected to both said signal device and said fluid circuit,whereby said switch is actuated to energize said signal device when thefluid pressure acting on said actuator approaches said predeterminedlevel.
 4. An actuator system according to claim 1, wherein said memberis the valve element of a slide gate valve.
 5. An actuator systemaccording to claim 1, wherein said member is the valve element of asemi-rotary butterfly valve.
 6. An actuator system according to claim 1,wherein said actuator includes a cylinder and a piston assembly which ismovable axially within said cylinder under the influence of fluidpressure and is connectable to said member to be actuated, said pressurerelief means being operable to prevent the pressure acting on saidpiston assembly reaching said predetermined level, said piston assemblydivides the interior of said cylinder transversely into two chambers,and said pressure relief means communicates with at least one of saidchambers when said cut-off means is inoperative.
 7. An actuator systemaccording to claim 6, wherein said cylinder includes inlet means forboth said chambers whereby pressurized fluid may be introduced to eitherof said chambers according to the desired direction of movement of thepiston assembly; and said pressure relief means is operable to relievepressure from whichever of said chambers is subjected to saidpressurized fluid, so long as said cut-off means remains inoperative. 8.An actuator system according to claim 7, wherein said cut-off meansincludes at least part of said piston assembly which, in said particularoperative position, is located to close communication between one ofsaid chambers and said relief means.
 9. An actuator according to claim8, wherein said piston assembly has a first said particular operativeposition in which said part thereof closes communication between one ofsaid chambers and said pressure relief means, and is positioned adjacentthe end of the cylinder remote from that chamber; and a second saidparticular operative position in which said part tHereof closescommunication between the other said chamber, and is positioned adjacentthe end of said cylinder opposite said firstmentioned end thereof. 10.An actuator system according to claim 9, wherein means is provided toreduce the effective end surface area of the piston assembly to whichpressurized fluid is applied to cause movement of the piston assembly inone direction, said means being arranged to cause said area reductionwhen said piston assembly is at or adjacent a said particular operativeposition; and the opposite effective end surface area of the pistonassembly to which pressurized fluid is applied for initially moving thepiston assembly in the opposite direction, is greater than said reducedarea.
 11. An actuator system according to claim 7, wherein a single portdefines both the inlet means and the outlet means for each said chamber,said ports communicating with their respective chambers through apressure operated control valve arranged to open both said ports totheir respective chambers in response to pressurized fluid being fedinto either one of said ports, whereby the port receiving saidpressurized fluid functions as inlet means for its respective chamberand the other said port functions as outlet means for its chamber. 12.An actuator according to claim 11, wherein said control valve is biasedto normally close communication between both said ports and theirrespective chambers, and safety valve means is connected to both saidchambers to prevent the pressure therein exceeding a predetermined upperlimit.
 13. An actuator system according to claim 6, wherein said pistonassembly includes a secondary piston which is formed by or attached toan end portion of a piston rod which forms at least part of saidconnecting means, and a primary piston which is slidably mounted on saidsecondary piston for limited axial movement relative thereto; saidpiston rod projecting through an end wall of said cylinder and beingmovable axially relative to the cylinder with said secondary piston;said pressure relief means includes a passage formed through saidsecondary piston to provide communication between said chambers; andsaid cut-off means includes at least part of said primary piston whichis arranged to open or close said communication according to its axialposition relative to said secondary piston.
 14. An actuator systemaccording to claim 13, wherein; said primary piston includes an innersleeve section slidable on said secondary piston, and an outer sleevesection which is slidable on said inner sleeve section; stop means isprovided on said secondary piston to limit axial movement of said innersleeve section relative to the secondary piston; and said outer sleevesection is located between opposed annular shoulders of said innersleeve section and is engageable therewith to limit its axial movementrelative to said inner sleeve section.
 15. An actuator system accordingto claim 14, wherein an abutment is provided within said cylinder forengagement by said outer sleeve section to limit axial movement of saidouter sleeve section relative to the cylinder in one direction; and saidinner sleeve section and secondary piston are able to move further insaid one direction after said outer sleeve section and said abutmentengage, whereby said outer sleeve section is located relative to saidsecondary piston to close said relief passage.
 16. An actuator systemaccording to claim 15, wherein said relief passage is closed when saidouter sleeve section engages both the said abutment and the said annularshoulder located remote from said abutment; and in that position of thepiston assembly the effective end area thereof which is engageable bypressurized fluid to move the said secondary piston further in said onedirection, is smaller than the effective opposite end area of the pistonassembly which is engageable by pressurized fluid to move said assemblyin the direction opposite to said one direction.
 17. An actuator systemaccoRding to claim 16, wherein two said passages are formed through saidsecondary piston, and said primary piston part is movable axiallyrelative to the secondary piston between two positions in each of whichsaid part closes a respective said passage.
 18. An actuator systemaccording to claim 17, wherein said outer sleeve section engages arespective said abutment in each of said two positions, and a reductionin the said effective end area of the piston assembly occurs in each ofsaid positions, the said end area reduced being opposite for each ofsaid positions.
 19. An actuator system according to claim 6, whereinsaid piston assembly has a first said particular operative position inwhich a first part of said piston assembly closes communication betweenone of said chambers and said pressure relief means, and is positionedadjacent the end of the cylinder remote from that chamber; and a secondsaid particular operative position in which a second part of said pistonassembly closes communication between the other said chamber and saidpressure relief means, and is positioned adjacent the end of saidcylinder opposite said firstmentioned end thereof.
 20. An actuatorsystem according to claim 6, wherein; said piston assembly includes twocylindrical end portions, an intermediate portion joining said endportions, and a rack formed on said intermediate portion and drivablyengageable with a pinion projecting through an aperture in a side wallof said cylinder, said rack and pinion forming at least part of saidconnecting means; said relief means includes a main relief passageproviding a communication between said chambers; and said cut-out meansincludes at least one of said end portions of the piston assembly whichis arranged to open or close said communication between said chambersaccording to the axial position of the piston assembly relative to saidcylinder.
 21. An actuator system according to claim 20, wherein saidmain relief passage is defined by a conduit located externally of saidcylinder and which communicates with each said chamber through arespective passage extending axially of the cylinder and arranged to beclosed by the adjacent end surface of the piston assembly.
 22. Anactuator system according to claim 21, wherein each said axial passageis formed in a stop element which projects axially into said cylinderfrom a respective end wall thereof, the end surface of each stop elementwhich is located within said cylinder serving as an abutment for theadjacent end of said piston assembly and having the respective saidaxial passage opening therethrough.
 23. An actuator system according toclaim 22, wherein each said stop element is adjustably mounted in therespective cylinder end wall to enable variation of the axial positionof the said end surface thereof.
 24. An actuator system according toclaim 20, wherein said relief means include relief valve means connectedto said main relief passage and operable to open and close communicationbetween said chambers through said main passage, according to the fluidpressure existing in said chambers.
 25. An actuator system according toclaim 24, wherein said relief valve means includes a valve housinghaving a compartment therein, said main passage communicating directwith said compartment and with one of said cylinder chambers, andcompartment inlet and outlet passages each of which communicates withthe other said cylinder chamber and is controlled by a respectiveone-way valve, the two said one-way valves being arranged torespectively allow fluid into and out of said compartment.
 26. Anactuator system according to claim 20, wherein said of the opposite endwalls of said cylinder is defined by a valve block which includes: acombined fluid supply and exhaust port, a pressure operable one-wayvalve which is normally biased into a closed position and controls fluidflow through said port, and an axially slidable spool element which ispressure operable to open said one-way valve; and rEspective drainpassage means connects the said port of each said valve block to a rearside of the said spool element of the other said valve block, wherebypressurized fluid supplied to one said chamber through the adjacent saidvalve block is operative to cause the said spool element of the othersaid valve block to open its respective said one-way valve and therebyallow said other chamber to exhaust.